The Tonight Show host made the comments after he showed his Tonight Show audience a photograph of Cowen.

He told the crowd that the man in the picture was not a bartender, but “the Prime Minister of Ireland” to uproarious applause.

Leno showed the dishevelled-looking Mr Cowen, asking the question: “Looking at that guy. How many think he is a bartender?”

He then asked if the audience thought he was a “nightclub comedian” or a “politician” – before revealing he was the “Prime Minister of Ireland” to loud applause and laughter.

“He's Brian Cowen, the Prime Minister of Ireland. Oh God, it's so nice to know we're not the only country with drunken morons, isn't it?” the star said.

Mr Cowen has been widely ridiculed since his infamous ‘Garglegate’ appearance on RTE's Morning Ireland radio show two weeks ago.

He gave a less-than-coherent performance and it emerged the Taoiseach had been up until after 3am the night before at the Fianna Fail think-in at the Ardilaun hotel in Galway.

The Tonight Show regularly attracts audiences of nearly four million viewers.

Impressionist Mario Rosenstock told a Sunday newspaper that Mr Cowen's reputation had been damaged and that he would begin to lose his “moral authority” if he continued to be the butt of jokes.

“When the Taoiseach starts becoming the butt of jokes, there is a problem. People will start taking his role and that of his office much less seriously,” said the man famous for Today FM satire Gift Grub.

“If the image of a drinker stays, then he will begin to lose his moral authority.

“This will get in the way of his job and undermine his credibility as a leader.”

He added: “All of that would have been aided by the fact the satire will have stuck to him.”

In the wake of the controversy over the radio interview, Mr Cowen promised to be more cautious in his social life.

He said he would have to be more careful about his behaviour in future.

“I think such is the atmosphere of politics today perhaps, and the way people interpret things and how things can go off on a tangent very quickly.

“I would be a bit more cautious in terms of that aspect of how I conduct my social life,” he said.

Minister for Foreign Affairs Micheal Martin conceded in the wake of the affair, which made headlines around the world, that it had been damaging to the Government.

“I think we have to really organise ourselves in a way that matches the mood of the people,” he said.